Complex compounds which contain, as a central atom, a metal of Group VIII of the Periodic Table of the Elements (IUPAC version) and, as ligands, P(III) compounds, phosphines, or phosphites, as well as possibly other groups capable of complex formation, have recently been gaining increased importance as catalysts. The reaction, performed industrially on a large scale, of olefins with synthesis gas to give aldehydes (hydroformylation) is carried out in the presence of catalyst systems which are composed of cobalt and, in particular, rhodium and triphenylphosphine. Catalysts based on phosphine-containing complex compounds have also proven themselves useful for the reaction of methanol with synthesis gas to give higher alcohols, in particular ethanol and propanol (homologization). The ligands are primarily present in excess in the cases mentioned, so that the catalyst system is composed of a complex compound and free ligand. In accordance with the solubility of the catalysts in organic media, the reactions are carried out in a homogeneous phase.
The reaction can also be carried out in a heterogeneous reaction system; an advantage of this process variant is the simple and mild separation of the catalyst, which is present in aqueous solution, from the water-insoluble reaction product. This principle is employed by, for example, the process described in DE-C2 27 00 904 for the addition of hydrogen cyanide to an unsaturated organic compound having at least one ethylenic double bond. For the preparation of aldehydes by reaction of olefins with carbon monoxide and hydrogen, according to the process of DE-C2 26 27 354, rhodium in metallic form, or in the form of compounds thereof, is used together with a water-soluble phosphine, e.g. the alkali metal salt of triphenylphosphine-trisulfonic acid ("TPPTS") as a catalyst.
The known two-phase processes have proven themselves very effective on an industrial scale. Nevertheless, efforts are being expended to improve the process even further. Thus, attempts are being made to increase and prolong the activity of the catalysts by modifying the complex ligands to reduce further the specific catalyst requirements--both rhodium and ligand--and hence the production costs. Economic grounds are also decisive for working towards a marked reduction of the phosphine/rhodium ratio. Furthermore, attempts are being made to develop specially adapted ligand systems which solve individual problems. As an example, the additional improvement in selectivity with respect to the formation of unbranched aldehydes can be mentioned. However, the opposite aim, the construction of branched-chain products, can also be of interest with respect to enantioselective syntheses. In this regard, it should be noted that several million tons of hydroformylation products are prepared annually, so that even a small change in selectivity in one direction or the other has economically significant consequences.